1. Field of the Invention
The present invention relates to a node connected to a serial bus and a bus power-supply device constituting the node suitable for IEEE-1394 Standards.
2. Description of the Related Art
In IEEE-1394 Standard information networks, as many as 63 of various nodes such as an audio visual apparatuses (AV apparatus) and a repeater, in addition to an information processing apparatus, can be connected on the same serial bus.
Then, an IEEE-1394 Standard serial bus includes a signal line for propagating a digital serial signal and a power-supply line. A node connected to the serial bus is therefore allowed to receive a supply of power from other nodes through a power-supply line to operate. As a result, IEEE-1394 Standard information networks have an advantage that not every node needs to be provided with an individual power source such as a commercial power-supply system. Hereinafter, an inherent power source each node individually has is also called “internal power source” for convenience' sake. On the other hand, a power source which is a direct-current (DC) voltage applied from other node through a serial bus is also called “bus power source”.
Here, one example of IEEE-1394 Standard information networks is shown in FIG. 5. FIG. 5 schematically shows an example in which five nodes 101, 102, 103, 104 and 105 are connected to one bus line 110. In this example, since the node 101 and the node 105 are supplied with a power-supply voltage from a commercial power-supply system, also to other nodes 102 to 104, power is supplied from the node 101 or the node 105 through the bus line 110. The nodes 102 to 104 therefore need not to be provided with an internal power source.
Next, description will be made of one node connected to a conventional IEEE-1394 Standard serial bus.
Although nodes, except a repeater, in general, have various components including a link layer in addition to main components such as a bus power-supply circuit, description will be made in the following with reference to each drawing having illustration only of main components for making the present invention easy to understand.
FIG. 6 shows an example of a structure of a node defined by IEEE-1394-1995. As shown in FIG. 6, a node 100c includes a power-supply circuit 2, a DC—DC 25 converter 5 and a physical (PHY) layer 6 which are connected to a serial bus through two connectors 8 and 9.
The connectors 8 and 9 include power-supply terminals 81 and 91 and signal terminals 82 and 92, respectively. To the power-supply terminals 81 and 91, a DC voltage is applied from a power-supply line of the serial bus. Furthermore, in the node 100c, the power-supply terminals 81 and 91 of the two connectors 8 and 9 are conductive to each other. The node 100c is therefore allowed to relay a DC voltage applied though the serial bus to other nodes.
On the other hand, to/from the signal terminals 82 and 92, signals are input/output from/to a signal line of the serial bus. Then, a signal input through one signal terminal of the two signal terminals 82 and 92 is subjected to data resynchronization by a local clock at the physical layer 6 and then output through the other signal terminal.
This arrangement enables relay of transmission and reception of signals to and from nodes not adjacent to each other on the serial bus line.
The power-supply circuit 2 is a means for supplying a DC voltage to the physical layer 6 and the serial bus which transmit and receive signals. The power-supply circuit 2 therefore converts a power-supply voltage input through a power-supply voltage input terminal 1 into a DC voltage suitable for the supply to the serial bus and outputs the DC voltage. The output DC voltage is applied to the power-supply terminals 81 and 91 through a diode 7 for preventing reverse current and to the DC—DC converter 5 as well.
To the DC—DC converter 5, a DC voltage is applied not only from the power-supply circuit 2 but also from the serial bus through the power-supply terminal 81. Then, the DC—DC converter 5 converts the applied DC voltage into a DC voltage suitable for the supply to the physical layer 6 and outputs the converted DC voltage. The output DC voltage is applied to the physical layer 6.
The node 100c is thus structured such that the physical layer 6 can be driven by either of a DC voltage applied from the bus power source through the serial bus and a DC voltage applied by the internal power source through the power-supply circuit 2.
To the serial bus, a DC voltage within a wide range from DC+8V to DC+40V can be applied according to the IEEE-1394 Standard. According to the IEEE-1394a Draft Standard which is an improvement of IEEE-1394, a DC voltage can be applied within a range from DC+20V to DC+33V.
As a result, there occurs a case where DC voltages applied to the serial bus from power-supply circuits 2 of various nodes connected to each other through the serial bus differ with the nodes. In this case, the highest DC voltage will be applied to the entire serial bus. A node which supplies the highest voltage should supply power to physical layers of all the nodes on the serial bus regardless whether the other nodes are supplied or not supplied with power from internal power supply systems.
In the information network using a serial bus shown in FIG. 5, in a case where both of the first and the second nodes 101 and 102 are supplied with a power supply voltage, when a supply voltage to the first node 101 is higher than that to the second node 102, the first node 101 should supply power also to the remaining nodes including the second node 102. In other words, although the second node 102 has an internal power source, power will be supplied from the first node 101 to the second node 102.
In order to solve this problem “IEEE-1394a Draft Standard” proposes an improvement in the structure of a bus power-supply device of a node.
Here, FIG. 7 shows an example of an improved structure of a node 10d. As illustrated in FIG. 7, although the improved node 100d has the same components as those of the above-described node 100c, it has different wiring paths connecting a power-supply circuit 2, a DC—DC converter 5 and two power-supply terminals 81 and 91.
More specifically, in the node 10d, a DC voltage output from the power-supply circuit 2 is applied to the power-supply terminal 81 though a diode 7 for preventing reverse current and to the power-supply terminal 91 as well through a diode 14.
On the other hand, a DC voltage applied through the power-supply terminal 81 is applied to the DC—DC converter 5 through a diode 11, while a DC voltage applied through the power-supply terminal 91 is applied to the DC—DC converter 5 through a diode 12.
As a result, at the node 100d, on the two wiring paths connecting the power-supply terminals 81 and 91, two diodes should be disposed in series in opposite directions to each other without fail. In the node 100d, therefore, no direct current will flow between the power-supply terminals 81 and 91.
On the path from the power-supply terminal 81 to the power-supply terminal 91 through the diodes 11 and 12, for example, current directing from the power-supply terminal 81 to the power-supply terminal 91 will not flow due to the diode 12 and current in the opposite direction will not flow due to the diode 11. On the path from the power-supply terminal 81 to the power-supply terminal 91 through the diodes 7 and 14, current directing from the power-supply terminal 81 to the power-supply terminal 91 will not flow due to the diode 7 and current in the opposite direction will not flow due to the diode 14.
The improved node 100d is thus structured such that a DC voltage is applied only to its adjacent node among all the nodes connected by the serial bus. It is accordingly possible to reduce the volume of power supply required of a node having the highest DC voltage.
With the improved node, however, no direct current flows between the power-supply terminals 81 and 91, so that relay of a DC voltage applied through the serial bus to other nodes is impossible. There accordingly occurs a problem that no DC voltage can be applied to other nodes than an adjacent node. In other words, the node has a restriction that to a node having no internal power-supply system, its adjacent node should supply power.
In the network shown in FIG. 5, for example, to the second node 102 having no internal power source, a DC voltage is applied from its adjacent first node 101 through the serial bus. To the fourth node 104, a DC voltage is applied from its adjacent fifth node 105 through the serial bus.
To the third node 103, however, a DC voltage will be applied from nowhere because the third node 103 and its adjacent nodes have no supply of a power-supply voltage. Therefore, the physical layer of the third node 103 will not be driven and the third node 103 is accordingly not allowed to relay a signal. As a result, signal transmission and reception is not allowed between the first and the second nodes 101 and 102 and the fourth and the fifth nodes 104 and 105. In other words, an information network can not be established using a serial bus.
Moreover, even in the improved node, when a power-supply voltage is supplied to both of nodes adjacent to each other, a highest DC voltage will be applied to both the nodes. As a result, there occurs a problem that one node which outputs a higher DC voltage should supply power also to the other node even if the other node has an internal power source.